1. Field of the Invention
The present invention relates to reflective mechanisms for deflecting light through a tape in a data cartridge.
2. Description of the Related Art
Tape in a data cartridge often is provided with holes to serve as codes to the drive in which the data cartridge is to be used. For example, the holes may indicate that the drive has reached the end or beginning of the tape, is nearly at the end or beginning of the tape, or may identify the exact type and length of tape in the cartridge inserted in the drive. The tape itself normally is opaque, and drives usually are provided with a light source and light detector to detect the presence or absence of such holes. Normally, the light is positioned above or below the cartridge and the detector to the front of the cartridge. The light shines through the top of the cover (which typically is transparent) or through a window cut through the metal baseplate of the data cartridge. It then reflects off of a mirror to redirect it about 90.degree. onto the tape. When the tape has a hole in it, the light passes through the hole and out through the front of the transparent cover, where it is detected by a photocell in the drive. In addition, the tape sometimes has multiple holes across its width, and a drive must be able to accurately determine the number of holes based on the amount or position of the light coming through the holes.
In some applications, a prism is substituted for the mirror. For example, co-pending U.S. patent application Ser. No. 08/346,777, filed Nov. 30, 1994, and titled "Prism for a Data Cartridge" describes a potential prism structure. Published European Patent Application 0 499 400 (Minnesota Mining and Manufacturing Company) describes another structure using a prism instead of a mirror, and notes various advantages to using a prism over a mirror.
In some applications, using a prism can pose problems. In certain cartridges, light usually is directed up through the bottom of the cartridge, then angled towards the tape (by the prism or a mirror) and detected at the front of the cartridge after passing through a hole in the tape. However, at least one drive manufacturer routinely directs the light backwards, that is, first shining it through the holes in the tape, then having it reflected toward the bottom of the cartridge, and then detecting it at the bottom of the cartridge. Moreover, the light source and the detector are off-center from the prism in the direction of tape travel. This technique works adequately when the cartridge uses a mirror to angle the light. However, it may cause problems if a prism is used due to extra reflections, e.g., from the sides of the prism, which can be detected as a phantom pulse. This is a problem especially in accurately detecting whether a single hole or multiple holes are present on the tape.
Co-pending U.S. application Ser. No. 08/429,427, filed Apr. 27, 1995, and titled "Prism for A Data Cartridge" describes a solution to this problem, namely, frosting portions of the prism surface from which reflection is not desired.
Frosted or not, a prism still has problems because it requires a fairly high mass of material. Specifically, the main body of the prism must be filled with material. This has several disadvantages:
First, the mere presence of more material increases the cost. It also usually takes longer to solidify, resulting in a long cycle time in a casting mold.
Second, most such prisms are produced by molding. Many types of thermoplastics are subject to shrinkage during solidification. Large masses of plastic tend to develop sinks. Depending on the location of the sink, this may affect, or even destroy, the desired optical characteristics of the prism.
These problems due to the mass of the prism body are avoided by using a mirror, e.g., as taught in Japanese Published Patent Applications Nos. 5-225,750, 5-225,751 and 5-225,752. However, mirrors have their own problems. It is difficult to make a mirror in which the mirror is silvered on the side closest to the tape. Primary reflection therefore usually is from the side the mirror away from the tape. This means that the light must pass through the front surface of the mirror before striking the primary reflecting surface, then return through the front surface of the mirror in its passage. Unfortunately, the front surface of the mirror also usually is angled, so it provides secondary reflectance. This results in the same phantom pulse problem noted with some prisms above.